The invention relates to a hydroformylation process.
Methods for producing aldehydes by the hydroformylation of an olefinically unsaturated organic compound with carbon monoxide and hydrogen in the presence of a rhodium-phosphorus complex catalyst and free phosphorus ligand are well known in the art, as evidenced by the low pressure oxo hydroformylation process of U.S. Pat. No. 3,527,809 and the rhodium catalyzed liquid recycle hydroformylation process of U.S. Pat. No. 4,148,830.
U.S. Pat. No. 3,527,809 discloses a hydroformylation process wherein olefinically unsaturated organic compounds are hydroformylated with carbon monoxide and hydrogen in the presence of a rhodium-phosphorus complex catalyst and free phosphorus ligand to produce aldehydes in high yields at low temperatures and low pressures. It is known that under such hydroformylation conditions, some of the product aldehydes undergo a condensation reaction to form higher-boiling, i.e., heavy, aldehyde condensation by-products, such as dimers, trimers and tetramers.
U.S. Pat. No. 4,148,830 discloses the use of these higher-boiling liquid aldehyde condensation by-products as a reaction solvent for the catalyst, which solvent also makes an excellent carrier for a continuous liquid recycle process. A continuous process removes from the reactor a liquid reaction effluent stream, which comprises the product, the solubilized catalyst, free phosphorus ligand and the higher-boiling aldehyde condensation by-products. The aldehyde product is then separated from the product solution by rapid volatilization in a vaporizer. The volatilized aldehyde product and the non-volatilized catalyst-containing liquid reaction solution is then disengaged in a gas-liquid separator, wherein the vaporized aldehyde product vapor stream is passed overhead through a condenser for recovery and the remaining non-volatilized catalyst containing liquid reaction solution is removed and recycled back to the reaction zone. A consideration in the design and operation of the vaporizer is the need to balance the rate of removal of the heavy condensation by-products, or heavies, with their rate of formation. If the heavies build up too much, the useful capacity and effectiveness of the hydroformylation system becomes reduced and the catalyst has to be replaced. This issue is reviewed in CN100522912.
Due to the sensitive nature of many ligands, the operation of the vaporizer must be designed to minimize time at elevated temperature. For example, in U.S. Pat. No. 4,166,773, it is preferred that the contact time at vaporizer temperatures be minimized, e.g., preferably less than 20 seconds. CN 1227190C teaches that the entire contact time in the vaporizer (including the heated tubes, cyclone, and gas-liquid separation section) should be less than 15 minutes.
An overly aggressive vaporizer operation can result in the loss of phosphorus ligand during the process due to the presence of volatilized phosphorus ligand in the vaporized aldehyde product. There is an economic penalty associated with heavies build-up and/or physical loss of the ligand. These problems can also lead to a need for further processing of the crude aldehyde product if deactivation of downstream aldehyde hydrogenation catalysts, which are employed in producing alcohols from the aldehyde, is to be prevented or at least minimized.
In a conventional, prior art vaporizer, such as the one shown in FIGS. 1a and 1b, the reaction fluid from the hydroformylation reaction zone is fed by stream (1) to the vaporizing zone (2) where the fluid is heated and the volatiles enter the gas phase, thus forming a mixture of gas and liquid phase materials. At the bottom of the vaporizing zone, this gas-liquid mixture enters the gas-liquid separation zone (3) where the volatile gases are separated from the non-volatilized material. The gases exit the gas-liquid separation zone via stream (4) through optional demisters (not shown), and other means to prevent entrainment of non-volatile materials, and are then cooled and collected downstream for further purification (not shown). The non-volatilized material is cooled by heat exchanger (6) and the cooled non-volatile material exits via stream (5) for further processing or to be returned directly to the reaction zone. In FIG. 1b, the heat exchange occurs in an external loop such that the non-volatile material is collected in the bottom of the gas-liquid separation zone. The liquid level, or gas-liquid interface level, (11) is maintained by varying the flows of streams (5a) and (5b). The liquid exits via stream (5) to the suction side of a pump (9), then is sent through a cooler (6) and is sent back to the reaction zone or to subsequent processing equipment (not shown) via stream (5b). A fraction of the cooled liquid can be returned to the gas-liquid separation zone via recycle stream (5a). Returning cooled liquid to the liquid region of the gas-liquid separation zone results in a lower temperature for the liquid region.
It is desirable to cool the non-volatile material in zone (3) as fast as possible since hydroformylation catalysts, especially phosphites and many phosphines, are thermally sensitive.
Unfortunately, this design results in a cold zone at the bottom of the gas-liquid separation zone (3). Thus, the desired product may condense at the gas-liquid interface (11) rather than escape the separation zone via stream (4). This results in a loss of efficiency as well as the buildup of aldehyde heavies (the least volatile material in the system). The cooled non-volatilized material in the separation zone may contain convection currents, which bring up cool fluid to the surface, thus cooling the gas liquid interface and exacerbating the problem.
It would be desirable to have a low-cost process wherein the catalyst-containing, non-volatile liquid material is quickly cooled without also condensing substantial amounts of the gaseous product stream.